Abstract

Submarine felsic volcanoes are dominated by hyaloclastic piles hundreds of meters
thick, the origin of which, in terms of how and when they form, is far from being completely understood.
Here we present a study of the thermal remanent magnetization of the Miocene high-K dacitic El Barronal
hyaloclastites (Cabo de Gata, Spain), showing that their formation is dominated by in situ fragmentation
with small or negligible transportation and/or rotation of different clasts after their formation. Data
indicate that fragmentation progressed down to 210–390 °C, well below the glass-transition temperature
estimated at 560–750 °C depending on the water content of the high-K dacite. Hence, hyaloclastite
fragmentation in thick lavas may occur over most of the cooling history, as a result of the progressive
access of sea water toward the lava interior by development of a complex network of contraction fractures.